Development and commercialization of the lithium ion battery has changed the entire complexion of the energy technology of portable electronics and power tools. Lithium ion batteries now power electric vehicles and are starting to replace gas backup generators with the introduction of the Powerwall. However, consumer desire for cell phones that last longer and lower cost electric vehicles (EVs) having greater driving range continuously fuels an on-going demand for higher energy batteries. This quest attracts interest towards finding a post lithium ion battery with 2-3 times the energy density now available with conventional lithium ion technology. Post lithium ion battery research includes effort to develop lithium/sulfur, lithium/air and multivalent metal batteries which operate with metallic anodes.
Commercial lithium ion batteries offer high stability upon cycling but also require the use of a host material in the anode which does not contribute to capacity. In order to increase the capacity of the anode, innovative methods must be adopted without sacrificing cycling stability and safety. For example, an insertion graphite anode with a specific capacity of 380 mAh/g may be upgraded to a lithium metallic anode with a specific capacity of 3660 mAh/g. Challenges associated with the use of lithium metal anodes are rooted in the high electronegative potential of lithium and a resultant high reactivity with battery electrolytes.
The reduction of electrolytes on the surface of the lithium results in the formation of a solid electrolyte interface (SEI). This SEI is unstable and non-uniform due to rupturing and reformation during the reversible plating of lithium which occurs during charge and discharge cycles. The rigid SEI cannot withstand the mechanical stress caused by the uneven nucleation and formation of dendrites or mossy deposits of lithium that form during the charging deposition. Growth of dendrites causes a rapid and large increase in the surface area of the anode and can cause thermal runaway or shorting of the battery. Upon each regeneration of the SEI, additional electrolyte is decomposed by reduction and as a result the coulombic efficiency between the deposition and dissolution of lithium is limited to below 50% after a few cycles at high rates. The low coulombic efficiencies impact cycle life of the battery negatively. For example, a 50% coulombic efficiency renders half the deposited lithium unrecoverable on each cycle and quickly depletes the lithium in the battery. In addition to this electrochemically inactive lithium, rapid degradation of the electrolyte which occurs during SEI regeneration also limits cycle life.
Therefore, for practical batteries with metallic anodes, a coulombic efficiency above 99% is desirable (less than 1% of lithium “lost” on each cycle). There have been no reports of electrolytes which are reductively stable on lithium metal and therefore, success in this technology may be obtainable with formation of a uniform and flexible SEI on the lithium metal anode. Recent approaches to stabilize the lithium metal anode and avoid dendrite formation include development of new electrolytes or additives which promote more uniform lithium electrodeposition. A related approach has been the ex-situ formation of the SEI by pretreatment with chlorosilanes, chlorophosphines, and chloroboranes, which appear to form protective layers on the lithium anode surface through reaction with Li. (Dunn et al. J. Mater. Chem., 2011, 21, 1593-1599) Solid electrolytes, polymers, ceramics and interconnected carbon (Cui et al., Nat. Nanotechnol., 2014, 9, 618-623) have also been explored to mitigate dendrite nucleation by blocking their growth. These solid state approaches suffer from interface issues which remain largely unresolved.
Thus, there is an ongoing need for a thin film lithium-electrolyte interface that is flexible and self-healing and can accommodate a large volume expansion during lithium deposition without rupturing. Such a protective film would impede electrolyte decomposition and provide for an electrochemical cell having enhanced coulombic efficiency.